The invention relates generally to a method and apparatus for manufacturing blocks of carbon dioxide snow which may be used for various applications including but not limited to cooling of shipping containers.
Dry ice has long been used for refrigeration of shipping containers. Its vaporization at a relatively low temperature without liquid residue and its light weight make it ideal for maintaining frozen food products and other products at acceptably low temperatures.
In refrigerated shipping containers which are cooled by dry ice, it is generally desirable to maximize the interior volume available for carrying product, while providing for the coolant only as much interior volume as necessary. In the past, there have been numerous efforts to provide high-density blocks of dry ice in order to maximize the refrigeration capacity that may be achieved for a given volume of dry ice.
As an alternative to high-density blocks of dry ice, lower-density CO.sub.2 snow may be "charged" into shipping containers. CO.sub.2 snow may be produced from liquid CO.sub.2 stored under pressure in a storage vessel, and discharged through a snow horn. Low-density CO.sub.2 snow may be produced more readily than high density blocks of dry ice. However, the low density CO.sub.2 snow is susceptible to rapid evaporation. Thus, it is generally desirable that charging of CO.sub.2 snow take place at or near the location at which it is produced.
In certain commercial operations such as distribution centers for fast food chains, one or more vessels of liquid carbon dioxide may be maintained for CO.sub.2 snow production, but shipping containers, particularly cabinet-style containers or other containers for small shipments, may often be loaded at locations remote from any of the vessels. Transportation of loose CO.sub.2 snow from a production location to the charging location results in vaporization losses, and charging by hand using scoops or the like typically results in further losses due to vaporization and spillage. Another problem is that it is difficult to assess production needs accurately in advance. This typically results in overproduction and waste. There is a need for a method and apparatus which enable solid CO.sub.2 to be produced quickly and at low cost, and charged into shipping containers without excessive sublimation losses, in commercial operations such as that described above and in other applications where relatively small and variable quantities of CO.sub.2 snow may be needed.